Effective, environmentally safe control of plant parasitic nematode infection is one of the largest unmet needs in crop protection. For example, it is estimated that nematodes annually cause soybean losses of approximately $3.2 billion worldwide (Barker et al., 1994) and that parasitic nematodes cost the horticulture and agriculture industries in excess of $78 billion worldwide a year, based on an estimated average 12% annual loss spread across all major crops. Therefore, improved methods for protecting plants from nematode infection are highly desirable since they would increase the amount and stability of food production.
Nematodes are active, flexible, elongate organisms that live on moist surfaces or in liquid environments, including films of water within soil and moist tissues within other organisms. Nematodes grow through a series of lifecycle stages and molts. Typically, there are five stages and four molts: egg stage; J1 (i.e. first juvenile stage); M1 (i.e. first molt); J2 (second juvenile stage; sometimes hatch from egg); M2; J3; M3; J4; M4; A (adult). Juvenile (“J”) stages are also sometimes referred to as larval (“L”) stages. Nematode parasites of plants can inhabit all parts of plants, including roots, developing flower buds, leaves, and stems.
There are numerous plant-parasitic nematode species, including various lesion nematodes (i.e. Pratylenchus spp.), root knot nematodes (i.e. Meloidogyne spp.), cyst nematodes (i.e. Heterodera spp.), dagger nematodes (i.e. Xiphinema spp.) and stem and bulb nematodes (i.e. Ditylenchus spp.), among others. However, the largest and most economically important groups of plant-parasitic nematodes are the families Pratylenchidae (lesion nematodes), Meloidogynidae (root knot nematodes) and Heteroderidae (cyst nematodes) with lesion and root knot nematodes being particularly noteworthy for their very broad host rages. Plant parasitic nematodes are classified on the basis of their feeding habits into the broad categories of migratory ectoparasites, migratory endoparasites, and sedentary endoparasites. Sedentary endoparasites, which include the root knot nematodes (Meloidogyne spp.) and cyst nematodes (Globodera and Heterodera spp.) induce feeding sites (“giant cells” in the case of root knot nematodes and “syncytia” for cyst nematodes) and establish long-term infections within roots. In contrast, while spending most of their lifecycles within host tissues, migratory endoparasitic nematodes like lesion neamtodes (Pratylenchus spp.) do not induce permanent feeding sites but feed while migrating between or through plant cells.
Traditional approaches to control plant diseases have relied on crop rotation, the construction of interspecific hybrids between resistant crops and their wild-type relatives as sources of resistant germplasm, and chemical treatment. However these traditional approaches all suffer from significant limitations in providing broad spectrum nematode control. Crop rotation or fallowing without weeding is not an effective strategy for controlling root lesion nematodes because of their broad host ranges which includes most crops, native grasses and weeds. Rotation is also less effective with the very broad host range Meloidogyne incognita, Meloidogyne javanica and Meloidogyne arenaria root knot nematodes. Genetic resistance is usually narrow spectrum (e.g., race specific in the case of cyst nematodes and species specific for lesion nematodes). Deployment of narrow resistance quickly results in race or species shifts in fields with nematode problems leading to loss of effectiveness of the resistant germplasm. Other challenges with genetic resistance include loss of potency at higher temperatures (e.g., Mi resistance to root knot nematodes) or reduction in the yields of elite germplasm when introgressing resistance traits from wild relatives.
In contrast, most chemical nematode control agents though broad spectrum, are not effective in eradicating nematode infestations. Nematodes deeper in the soil or inside roots are largely protected and can cause significant crop damage later in the growing season. The few agents like the fumigant methyl bromide that can effectively get to nematode reservoirs are biocides effectively sterilizing a field for a period of time. Furthermore, methyl bromide, which was once the most widely used fumigant nematicide, is scheduled to be soon retired from use, and at present there are very few, if any, promising candidate to replace this treatment. The non-fumigant organophosphate and carbamate nematicides like ethoprop, terbufos, carbofuran and aldicarb though not as broad spectrum also show poor selectivity. In particular these chemical nematode control agents are highly toxic to mammals, birds, fish, and to non-target beneficial insects. These agents can in some cases accumulate in the water table, the food chain, and in higher trophic level species. These agents may also act as mutagens and/or carcinogens to cause irreversible and deleterious genetic modifications. As a result, government restrictions have been imposed on the use of these chemicals. Additionally, few chemical nematicides (fumigant or non-fumigant) are cost effective for use in large acreage row crops such as soybeans and corn. There has been renewed interest recently in chemical seed treatments which can be economically applied in large acreage row crops but these only provide early season protection under moderate levels of nematode infestation.
In addition to nematode pests, plants are typically subject to multiple disease causing agents such as fungi and insects which often potentiate the effect of the nematode. Examples of these disease complexes include the Fusarium solani gal/soybean cyst nematode pairing in soybean sudden death syndrome and the rootknot nematode/fursarium wilt complex in cotton. Therefore methods of controlling nematodes having broader pesticidal effects are particularly desirable.
The methods of plant biotechnology have been shown to provide an effective means to control insect infestations by having the plant express an insect control agent. However, there are few examples of effectively applied biotechnology methods to simultaneously control nematode and other plant pathogens such as insects and fungi.